Anecdotally I was walking half a mile to school in the second grade on my own. My nieces and nephews are the same age and aren't allowed to leave their yard (despite having cell phones!).
There's also the question of "why" — kids today are adept at video calls and tend to play lots of video games (many of which only have online multiplayer modes anyway!). They're generally not spending a lot of time playing outside. Their parents are also spending more time inside.
I'm sure there are a preponderance of reasons for the changes, but I've always suspected that the access to an infinite news cycle (e.g., some kid 2 hours away went missing last night) has made people overly cautious. This could be entirely anecdotal, but I feel like the world was a lot smaller for parents before the internet and this has altered perceptions (despite the fact that most places are actually safer in terms of violent crime rates than they used to be).